The invention relates to an optical semiconductor device and a process for producing the same. More particularly, the invention relates to an optical semiconductor device for use in WDM (wavelength division multiplexing) transmission, comprising a plurality of semiconductor lasers with different oscillation wavelengths which have been simultaneously formed on a single substrate, and a process for producing the same.
In recent years, attention has been drawn to optical communication as means for coping with a rapidly increasing demand for communication. In this connection, a WDM optical communication system has attracted attention, because this system can transmit optical signals with different wavelengths over a single optical fiber to realize high capacity communication over a single optical fiber. WDM is a system wherein, on a transmitter side, a plurality of different wavelengths (n wavelengths: xcex1 to xcexn) are multiplexed in an optical multiplexer to perform wavelength multiplexing on a single optical fiber, followed by transmission, while on a receiver side, information from the optical fiber is demultiplexed in an optical demultiplexer to wavelengths xcex1 to xcexn. The realization of WDM requires, for example, the stabilization of laser wavelengths, the development of optical circuit devices, and the integration of optical circuits. Further, in the WDM optical communication system, a plurality of light sources corresponding respectively to the wavelengths xcex1 to xcexn are required. Accordingly, what is required here is to efficiently realize light sources respectively with different wavelengths. To cope with this, Japanese Patent Laid-Open No. 117040/1998 discloses a production process wherein a plurality of DFB (distributed feedback) lasers with different wavelengths and a plurality of EA (electro-absorption) modulators, which have been integrated with each other, are simultaneously prepared within a plane of a single semiconductor substrate. According to the production process disclosed in Japanese Patent Laid-Open No. 117040/1998, in order to provide a plurality of different oscillation wavelengths on a single semiconductor substrate, diffraction gratings with different periods (pitches A1, A2 . . . An) are formed by electron beam exposure and etching, and multi-layer structures each comprising an active layer (a light absorption layer) with a band gap wavelength according to the oscillation wavelength are then prepared by selective MOVPE (metal-organic vapor phase epitaxy).
FIG. 33 shows an optical semiconductor device disclosed in Japanese Patent Laid-Open No. 117040/1999. As shown in FIG. 33A, at the outset, 16 phase-shift diffraction gratings 302 with different periods are successively formed on an n-type InP substrate 301 in its stripe region by electron beam (EB) exposure and etching. The pitch of the diffraction gratings 302 is varied to xcex1, xcex2, and xcex3 in the order of an increase in the pitch. Next, a striped SiO2 mask 303 as an insulation layer for selective growth is formed in the direction of [011]. This SiO2 mask 303 is formed on the diffraction grating 302 so as to have a striped window region having a predetermined width. In this case, the mask width is varied according to the pitch of the diffraction grating 302. Thereafter, as shown in FIG. 33B, an n-type InGaAsP guide layer 304, an InGaAsP/InGaAsP-MQW (multiple quantum well) active layer 305, and a p-type InP cladding 306 are formed in that order by selective MOVPE growth in a region between the SiO2 masks 303. The multilayer semiconductor layer formed in the region between the SiO2 masks 303 functions as an optical waveguide. In FIG. 33B, a semiconductor layer formed between masks with different widths is not shown in the drawing for the simplification of the structure.
Next, the opening width of the SiO2 mask 303 around the striped optical waveguide is widened. Thereafter, as shown in FIG. 33C, a p-type InP buried layer 307 is formed again by selective MOVPE growth. In FIG. 33C, as with FIG. 33B, the semiconductor layer formed between the SiO2 masks 303 with different widths is not shown in the drawing.
Next, an SiO2 layer 300 is formed on the assembly except for the top of the ridge structural multilayer semiconductor including the MQW active layer 305, and, as shown in FIG. 33D, a metal electrode 309 is formed on the surface of the InP substrate 301, while a metal electrode 310 is formed on the backside of the InP substrate 301. The metal electrode 309 formed on the surface of the InP substrate 301 is separated for insulation between the devices. Thereafter, the wafer is cleaved at an interval of the semiconductor laser device length. An antireflection (AR) coating is applied onto the cleaved end face to complete semiconductor lasers.
According to the process shown in FIG. 33, the band gap wavelength of the oscillation wavelength can be made identical to the band gap wavelength of the laser active layer in a given range (detuning). Therefore, this process features that the homogeneity of the threshold of laser oscillation and the oscillation efficiency can be kept relatively good. The preparation of the active layer by selective MOVPE, however, leads to a change in band gap wavelength of the optical guide layer formed on the diffraction grating and, in addition, a change in thickness of the active layer. The change in the band gap wavelength of the guide layer leads to a change in absolute value of the refractive index on the diffraction grating. This in turn leads to a change in level of a periodic refractive index change by diffraction gratings. Further, a change in thickness of the active layer leads to a change in an optical confinement factor in the active layer. This results in a change in light intensity in the diffraction grating region. The level of the periodic refractive index change due to the diffraction grating and the light intensity of the diffraction grating region are parameters involved directly in coupling coefficient xcexa (which represents the relationship of coupling where reflection occurs in the diffraction grating to couple a traveling wave with a back wave and is a parameter as an index for resonant characteristics). Therefore, in the simultaneous formation of lasers with different wavelengths wherein the level of the refractive index change and the light intensity in the diffraction grating region are varied, the coupling coefficient xcexa is varied according to the oscillation wavelength.
Here an optical semiconductor device produced by the production process shown in FIG. 33 will be discussed. (1) The increase in the widths (Wnm1 to Wnm3) of the masks for selective growth to increase the oscillation wavelength increases the band gap wavelength of the optical guide layer on the diffraction grating. This increases the absolute value of the refractive index of the optical guide layer. As a result, the coupling coefficient xcexa is increased. (2) Since the thickness of the active layer is increased, the coefficient of light confining in the active layer is increased. Thus, the light intensity in the diffraction grating region is reduced, and the coupling coefficient xcexa is reduced. The relationship between the oscillation wavelength (or the width of mask for selective growth) and the coupling coefficient xcexa varies depending upon the relationship between the magnitude in (1) and the magnitude in (2) (which depends upon an MOVPE apparatus for growth of crystal or growth conditions). This coupling coefficient xcexa is a parameter which is closely related, for example, to the oscillation threshold of DFB laser, luminous efficiency, yield of longitudinal single mode, and long-distance transmission characteristics.
According to the conventional optical semiconductor device and production process, however, since the coupling coefficient xcexa varies for each DFB oscillation wavelength, the threshold current of laser oscillation and the luminous efficiency become heterogeneous. This heterogeneity results in lowered yield of devices.
Further, since the coupling coefficient xcexa varies for each DFB, the resistance to the residual reflection of the end face varies from device to device. This poses a problem that, in the case of long-distance transmission, the yield of transmission characteristics varies according to the wavelength.
Accordingly, it is an object of the invention to provide an optical semiconductor device and a process for producing the same which can suppress a variation in coupling coefficient caused by the simultaneous formation of a plurality of semiconductor lasers on a single semiconductor substrate, can homogenize the threshold of laser oscillation, luminous efficiency, and long-distance transmission characteristics, and can improve the yield.
In order to attain the above object of the invention, according to the first feature of the invention, an optical semiconductor device comprises a plurality of semiconductor lasers which oscillate longitudinal single mode laser beams based on a periodic change in refractive index or a periodic change in gain and have been simultaneously formed with mutually different oscillation wavelengths on a single substrate,
said plurality of semiconductor lasers being identical to each other in coupling coefficient independently of the oscillation wavelength.
By virtue of this construction; unlike the conventional optical semiconductor device which comprises semiconductor lasers, having an identical construction, simultaneously formed on a single substrate and varies in coupling coefficient for each oscillation wavelength, for the optical semiconductor device of the invention, the coupling coefficient of the semiconductor lasers is identical despite the fact that the semiconductor lasers are different from each other in oscillation wavelength. Therefore, the optical semiconductor device of the invention is free from the heterogeneity of the threshold current of laser oscillation and the luminous efficiency, and, thus, the lowering in yield of semiconductor laser devices can be prevented. Further, in long-distance transmission, a variation in yield of the transmission characteristics depending upon the wavelength can be prevented.
In order to attain the above object of the invention, according to the second feature of the invention, an optical semiconductor device comprises a plurality of semiconductor lasers which oscillate longitudinal single mode laser beams based on a periodic change in refractive index or a periodic change in gain and have been simultaneously formed with mutually different oscillation wavelengths on a single substrate,
said plurality of semiconductor lasers being provided with diffraction gratings having heights corresponding respectively to the oscillation wavelengths.
According to this construction, the height of the diffraction grating is regulated in such a manner that, despite the fact that the semiconductor lasers are different from each other in oscillation wavelength, the coupling coefficient of the semiconductor lasers is identical. As a result, since the coupling coefficient of the semiconductor lasers is identical despite the fact that the semiconductor lasers are different from each other in oscillation wavelength, the optical semiconductor device is free from the heterogeneity of the threshold current of laser oscillation and the luminous efficiency, and, thus, the lowering in yield of semiconductor laser devices can be prevented. Further, in long-distance transmission, a variation in yield of the transmission characteristics depending upon the wavelength can be prevented.
In order to attain the above object of the invention, according to the third feature of the invention, a process for producing an optical semiconductor device comprising a plurality of semiconductor lasers which oscillate longitudinal single mode laser beams with different wavelengths and have been simultaneously formed on a single substrate, comprises the steps of:
coating a resist on the substrate;
exposing the surface of the resist to a pattern of a plurality of diffraction gratings for setting pitches corresponding respectively to oscillation wavelengths for the plurality of semiconductor lasers and for setting heights which provide an identical coupling coefficient independently of the oscillation wavelength;
etching the coating in such a manner that the level of etching per unit time is identical;
patterning a stripe mask to give a predetermined shape according to the arrangement of the diffraction gratings;
forming a laser active layer an each ofxe2x80x2 the diffraction gratings using the SiO2 mask having a predetermined shape by selective MOVPE growth (metal-organic vapor phase epitaxy);
forming an electrode on each of the top surface of the laser active layer and the backside of the substrate.
According to this production process, in forming a plurality of semiconductor lasers with a plurality of diffraction gratings on a substrate, the height of each diffraction grating is set so that the coupling coefficient is identical for the semiconductor lasers, followed by the formation of laser active layers respectively on the diffraction gratings. By virtue of the control of the height of the diffraction gratings, for the diffraction gratings on the single substrate, the coupling coefficient is identical. Therefore, the optical semiconductor device is free from the heterogeneity of the threshold current of laser oscillation and the luminous efficiency, and, thus, the lowering in yield of semiconductor laser devices can be prevented. Further, in long-distance transmission, a variation in yield of the transmission characteristics depending upon the wavelength can be prevented.